Method of extruding bi-helically oriented thermoplastic tube



DONALD METHOD OF EXTRUDING BI-HELICALL'I' ORIENTED Oct. l, 1968 H. J.

THERMOPLASTIC TUBE Filed Jan. 28, 1965 1 N VEN TOR. Haro/a/Jack Oona/a ZZZ 15 United States Patent O 3,404,203 METHOD OF EXTRUDIN G BI-HELICALLYORIENTE!) THERMOPLASTIC TUBE Harold Jack Donald, Midland, Mich.,assignor to The Dow Chemical Company, Midland, Mich., a corporation ofDelaware Continuation-impart of application Ser. No. 363,039,

Apr. 3, 1964. This application Jan. 28, 1965, Ser.

4 Claims. (Cl. 264-108) This application is a continuation-in-part of myprior application Ser. No. 363,039, filed Apr. 3, 1964, now abandoned,which in turn was a divisional application of my prior application Ser.No. 277,827, iiled May 3, 1963, also now abandoned.

This invention relates to the extrusion of thermoplastic resinousmaterials. It more particularly relates to the extrusion of hollowcylindrical articles such as pipe, tube, and the like.

Considerable difliculty has been encountered in the preparation ofplastic tube and pipe with regard to the physical strength of theresultant product. In order to achieve tube and pipe of adequatestrength for many practical purposes it has been necessary to utilize awall thickness substantially in excess of that which might be reasonablyrequired from an examination of physical properties of the plasticemployed. Therefore, it has been necessary in order to provide adequatestrength to prepare tubes and pipes which had thicker wall sections thanmight ordinarily be desired from an economical standpoint as well asweight standpoint. The process of extrusion frequently incorporateswithin the walls of the pipe or tube inherent defects or weak areas.Generally such weakened regions comprise linear faults extendinglengthwise along the conduit wall and are frequently caused by weldlines. For example, in the extrusion of the tube about a mandrelfrequently the plastic material is forced into the extruder die in sucha manner that it must be divided and flow about the mandrel. Thus thepoint at which the incoming stream flows together again often forms whatis known as a weld line within the extruded product. Frequently such aweld line is a region of weakness. Oftentimes the internal mandrel of atubing or pipe die is maintained in centered position by means of aplurality of arm-like supports generally referred to as a spider. Thesearms extend from the mandrel to the internal walls of the extruderbarrel or die and each of the arms is in the path of flow of thethermoplastic resinous material and as the material separates and ilowsabout the arms of the spider a weld line is usually generated.Frequently plastic pipe in service will fail by longitudinal rupture dueto internal pressure and oftentimes when utilized as a structural memberwill exhibit a similar type of failure when an external crushing forceor bending moment is applied.

It is an object of this invention to provide an improved method for theextrusion of thermoplastic pipe and like articles.

Another object of this invention is to provide an improved method ofmaking reinforced thermoplastic resinous tubular article.

A further object of this invention is to provide a method of making anextruded thermoplastic resinous tubular article having improved physicalcharacteristics.

Still another object of this invention is to provide a method of makingan extruded thermoplastic resinuous pipe having more uniform physicalproperties.

These benefits and other advantages in accordance with the presentinvention are achieved in a method for the extrusion of thermoplasticresinous pipe wherein a heat plastied thermoplastic resinous material isextruded ice through an annular orifice to form a hollow cylindricalbody, the improvement which comprises imparting orientation to theextruded body in a direction other than the direction of extrusion andhaving the direction of orientation of the inner and outer surfacesdiifering from each other.

A particularly advantageous embodiment of the invention is achieved whena thermoplastic resinous material containing lamentary reinforcing isemployed in the preparation of such a hollow tubular article.

Further features and advantages of the present invention will becomemore apparent from the following specication when taken in, connectionwith the drawing wherein:

FIGURE 1 is a schematic sectional representation of extrusion inaccordance with the present invention;

FIGURE 2 is an end view .of the annular die of FIG- URE 1;

FIGURE 3 is a schematic isometric representation of a tube or pipe ofthe present invention.

In FIGURE 1 there is schematically represented an extrusion apparatusgenerally designated by the reference numeral 10. The apparatus 10comprises an extruder barrel 11. Disposed Within the extruder barrel 11is an extruder screw 13 forwarding a heat plastied thermoplasticresinous material 14. A crosshead 15 is aixed to the terminal portion ofthe extruder barrel 11. The crosshead 15 is provided with an internalpassageway 16 within which there is disposed a rotating mandrel 18. Therotating mandrel 18 is rotatably supported by a crosshead bearing 19. Ameans to rotate the mandrel 18 is affixed to the portion 20 of themandrel extending beyond the crosshead 15. Aixed to the portion 20 ofthe mandrel 18 is a spur gear 21, which is in operative engagement witha pinion gear 22, which in turn is rotated by the gear head motor 23.Opposite the crosshead bearing 19 the crosshead 15 terminates in abearing flange 12. A die 24 is rotatably secured to the bearing flange12 by means of the retainer 25 aixed to the die 24. Means to rotate thedie 24 comprises a spur gear 26 integral with the body of the die 24which is in operative engagement with the pinion gear 28. The piniongear 28 is' in turn rotated by the gear head motor 29. The die 24 andthe mandrel 18 in combination define an annular passageway 31 throughwhich the thermoplastic resinous material 14 is extruded to form thetubular article 33. Beneficially a temperature control jacket 35 may bebeneficially disposed about the crosshead 15.

In FIGURE 2 there is illustrated an end view of the apparatus 10 takenalong the line 2--2 of FIGURE 1 showing the relative direction ofrotation of the die 24 and the mandrel 18.

In FIGURE 3 there is illustrated an isometric representation of athermoplastic resinous tube in accordance with the invention generallydesignated by the reference numeral 35. The tube 35 defines an innergenerally cylindrical surface 37, an outer cylindrical surface 38 and awall 39. The arrows on the surface 38 designate the direction oforientation within that surface, whereas the arrows situated on thesurface 37 similarly indicate the direction of orientation of thatsurface. The method of the present invention can be best understood byreference to FIG- URES l, 2 and 3 wherein a thermoplastic resinousmaterial is extruded at suitable extrusion temperatures to form a tubesubstantially in the manner of conventional extrusion apparatus with theexception that the die 24 and the mandrel 18 are rotated in oppositedirections in such a manner as to effectively twist the internal andexternal surfaces of the tube in opposite circumferential directionsgiving rise to a resultant product which has non-linear orientation. Itis critical in the practice of the present invention that neither themandrel-plastic interface nor the die-plastic interface be lubricated toa degree sufficient to prevent the interior or exterior surfaces of theplastic tube from being rotated with respect to each other. Thuseffectively the wall of the plastic Iarticle being extruded is placedunder a circumferential shear force as well as the linear shear forcegenerated by the expulsion of the plastic material from the die. Theresultant orientation pattern of such a tube, for example, if producedutilizing1 the die rotation as shown in FIGURE 2 would be to provide ahelically oriented interior surface having a generally right handhelical pattern. The external surface would be oriented in a left handedhelical pattern. The portions of the wall lying between the two surfacesare oriented in a manner generally lying between the orientation patternof the outer surface and the inner surface and in general havingsomewhat less of a helical pattern than either of the surfaces.Extrusion temperatures and conditions for the practice of the presentinvention in general are substantially similar to those employing theconventional non-rotating non-orienting dies or the conventionalrotating non-orienting dies. Oftentimes the temperature within thecrosshead may be maintained slightly lower than in conventionalprocesses because of the minor amount of heat generated in the extruderby the rotational shear imparted to the plastic by the rotating die andmandrel. Pipes and tubes extruded in accordance with the foregoingdescription are found to have substantially higher burst strengths, beamstrengths and compressional strength than pipe extruded in accordancewith known practice and without surface orientation. Although theinvention has been described utilizing both a rotating mandrel and arotating die, it is readily apparent that significant benefits andadvantages are achieved by the orientation introduced either byutilizing a rotating die or rotating mandrel. When this technique isutilized a single helical pattern is generated. For example, if in theapparatus of FIGURE 1 the die 24 is stationary and the mandrel 18 isrotated, a right handed helical orientation would occur within the pipewhere the inner surface would be provided with a helical orientation andthe external surface will have a linear orientation. This embodiment isparticularly advantageous wherein the pipe or tube will be underexternal radial compressive force with a rotating external die. Thehelical orientation will be at a maximum at the external surface andwill be reduced to about zero at the internal surface. This embodimentis particularly useful wherein the pipe is adapted to withstand internalpressure. However, most advantageous is the embodiment wherein helicalorientation is applied both to the internal surface and to the externalsurface and the orientation is of opposite hand.

Generally in the practice of the present invention it is desirable toorient one surface of the pipe to a sufficient degree that the helicalpattern covers about 180 of the cylindrical surface in a length of about5 diameters. Beneficially, for maximum strength in all directions,orientation of the surfaces should be about 360 in a distance of 4diameters. For example, if a tube is 3 inches in diameter theorientation would be l turn per foot.

The method of the present invention is readily practiced using a widevariety of thermoplastic resinous materials including polyvinylchloride,polyethylene, polypropylene, resinous copolymers of ethylene andpropylene, polystyrene, copolymers of styrene and other monomers such asstyrene and acrylonitrile, styrene methyl acrylate and the like. Alsobeneficially employed are polymethyl methacrylate, and copolymers ofmethyl acrylate and other monomeric materials copolymerizable therewith,linear polyamides such as those prepared by the condensation ofhexamethylene diamine and adipic acid, polycaprolactam, and the like.

Particularly advantageous and beneficial bodies in accordance with theinvention are prepared when filamentary reinforcing materials areemployed such as, for example, glass fiber and the like. Typicalthermoplastic resinous materials which may be utilized include thealkenyl aromatic resins typified by polystyrene, styrene copolymers,blends and graft copolymers of styrene and rubber and the like. Theinvention is readily practiced utilizing polyvinylchloride, vinylidenechloride copolymers such `as are generally shown as Sarans,superpolyamides such as Nylon 66 (a condensation product ofhexamethylene diamine and adipic acid), the polyolens includingpolyethylene, polypropylene and resinous copolymers thereof, ethylcellulose, cellulose acetate, rubbers both natural and syntheticincluding polybutadiene, polyisoprene and including the chlorinatedderivatives, mixtures thereof, and the like.

A wide variety of filamentary reinforcing agents 4may be utilizedincluding certain thermoplastic materials when utilized with otherresinous materials which have a significantly lower heat-formingtemperature than does the reinforcing material. Particularlyadvantageous and beneficial are the thermoplastic resinous compositionsutilizing filamentary glass or Fiberglas as a reinforcing medium.Beneficially such filaments are used in lengths from about 0.05 inch toabout 2 or more inches depending upon the characteristics of the productdesired and the equipment available. Usually, however, the advantageousrange is from about 0.1 inch to about 0.75 inch. Polycarbonate resinsare also beneficially employed. The particularly suitable polymers foruse in the present invention are those that, when heat plastified, havea viscosity of between about 300 poises and about 106 po-ises. If theviscosity range is below about 300 poises, generally the filamentarymaterial is prone to separate under the inliuence of gravity and theclearances required for pumping and forwarding in an extruder become soclose that considerable vfiber damage is done and a beneficial productdoes not result. If the heat plastified resin has a viscosity greaterthan about 106 poises, the stresses placed on the reinforcing `materialare often too great and significant strength is lost in the resultantproduct. Even `with the lower viscosity materials, regions of high shearmust -be avoided. Other factors may reduce the reinforcing filaments inlength such as, for example, a small extrusion orifice wherein thepolymeric material is being extruded under conditions which causeturbulence. If the polymer is required to pass over a sharp edge, suchas a rectangular edge of a hole formed normal to the die plate, thefibers adjacent the sharp edges are subjected to severe shear. When suchfilamentary reinforced materials are employed to prepare tubulararticles in accordance with the invention as presently claimed, thefilaments are oriented in generally the same direction and manner as thepolymeric material forming the tbody and the reinforcing fibers orfilaments of the inner surface are substantially parallel to each other,those of the outer surface are substantially parallel to each other, andthe bers of the outer surface and inner surface are disposed in angularrelationship, greatly increasing the burst strength of the pipe over andabove that of an unoriented imateriaL For example, a two inchelectrically heated extruder was fed with a polyethylene admixed with 10weight percent of 1A inch long chopped glass fiber roving, and extrudedto form a tube 2 inches in diameter and having a quarter inch wallthickness. The tube was extruded using an internal mandrel rotation rateof 10 revolutions per minute and an external rate of l0 revolutions perminute in the opposite direction. The extrude was passed to a water bathhaving a temperature of 17 centigrade spaced about 3 inches from adownwardly extruded die. A sample of the pipe had a burst strength of6,700 pounds per square inch.

The foregoing experiment is repeated with the exception that the diesare not rotated. A substantially lower burst strength is obtained.

As is apparent from the foregoing specification, the method andmanufacture of the present invention are susceptible of -being embodiedwith various alterations and modifications which lmay differparticularly from those that have been described in the precedingspecification and description. For this reason, it is to be fullyunderstood that all of the foregoing is intended to be merelyillustrative and is not to be construed or interpreted as beingrestrictive or otherwise limiting of the present invention, excepting asit is set forth 4and defined in the hereto appended claims.

What is claimed is: 1. In a method for the extrusion of a thermoplasticresinous tube comprising extruding a heat plastifed thermoplasticresinous Inaterial through an annular die, the annular die orifice beingdefined by inner and outer orifice walls, and subsequently cooling theheat plastied thermoplastic materialybelow its heat plastifyingtemperature to provide -a -generally cylindrical tube having a diameterabout equal to the diameter of the die orifice, the improvement whichcomprises rotating the orifice walls in opposite directions thereby andmolecularly orienting the heat plastified thermoplastic materialadjacent the inner and outer walls as it is discharged from theextrusion orifice in a generally helical manner, the helical pattern oforientation adjacent the inner surface of the tube being of oppositehand than the helical pattern adjacent the outer surface.

2. The method of claim 1 including the step of helically molecularlyorienting the surface of the pipe at least about for a length of about 5diameters of the pipe.

3. The method of claim 1 including the step of molecularly orienting thepipe adjacent the surface about 360 in a lineal distance 0f about 4diameters of the pipe.

4. The method of claim 1 including the step of providing a thermoplasticresinous material containing a lilamentary reinforcing agent for thepreparation of the resinous tube.

References Cited UNITED STATES PATENTS 2,676,356 4/ 1954 Becker 18-122,803,042 8/ 1957 Lundh 18-12 3,020,588 2/ 1962 Ferguson et al 18-143,051,990 9/1962 Peterson 18-14 3,059,277 10/1962 Pierce et al. 18-123,180,909 4/ 1965 Looser 18-14 3,223,762 12/ 1965 Fry 18-14 3,244,781 4/1966 Covington et al 264-209 3,008,187 11/ 1961 Slade 264-299 ROB'ERT F.WHITE, Primary Examiner.

T. J. CARVIS, Assistant Examiner.

1. IN A METHOD FOR THE EXTRUSION OF A THERMOPLASTIC RESINOUS TUBECOMPRISING EXTRUDING A HEAT PLASTIFIED THERMOPLASTIC RESINOUS MATERIALTHROUGH AN ANNULAR DIE, THE ANNULAR DIE ORIFICE BEING DEFINED BY INNERAND OUTER ORIFICE WALLS, AND SUBSEQUENTLY COOLING THE HEAT PLASTIFIEDTHERMOPLASTIC MATERIAL BELOW ITS HEAT PLASTIFYING TEMPERATURE TO PROVIDEA GENERALLY CYLINDRICAL TUBE HAVING A DIAMETER ABOUT EQUAL TO THEDIAMETER OF THE DIE ORIFICE, THE IMPROVEMENT WHICH COMPRISES ROTATINGTHE ORIFICE WALLS IN OPPOSITE DIRECTIONS THEREBY AND MOLECULARLYORIENTING THE HEAT